Easy to create functional wireframes even if you don’t know how to code.

When POs need to see something fast, it's good.

I LOVE Axure. I cannot say enough good things about it. I often create prototypes for accounting problems, and it is incredibly important that our testing participants are able to actually type into the prototype and see it react. Axure is invaluable for creating these prototypes. No other prototyping tool that I have used adds interactive text entry fields in seconds. For more complex prototypes, you can add logic, including calculations, pushing/pulling widgets, and changing text in other places on the prototype. "Masters" mean that you can consistently implement navigation or other static items with ease.